Research Paints New View of Tropical Forests

A global study of the deep evolutionary history of tropical forest plant species that includes research by College of the Atlantic plant sciences professor Dr. Susan Letcher is providing a new perspective on the major divisions between tropical forests around the world.

BY LEELOU GORDON-FOX ’21

“Phylogenetic classification of the world’s tropical forests” published in theProceedings of the National Academy of Sciences of the USA (PNAS), found that South American and African forests actually share similar ancestry, as do Eurasian tropical forests and North American subtropical forests.

Dr. Susan Letcher is a professor of plant sciences at College of the Atlantic.Through studying common ancestry of flora across the globe, the study demonstrated, for example, that a dry forest in India may be more similar to a central American dry forest than to a neighboring wet forest also in India.

“Such findings illuminate scientists’ biogeographical knowledge, as we are now able to trace where the American and African continents were ripped apart in the signature of tropical forest DNA,” Letcher said.

Led by author Ferry Slik, Professor in Plant Geography at theUniversiti Brunei Darussalam,the study in macro-ecology compiles data sets about tropical and sub-tropical forest flora diversity recorded by 200 scientists. It takes a new approach to taxonomy–rather than just classifying the flora of all the tropical forests worldwide at a general species level, Slik looks at the deeper scale of connectivity of those flora.

“The evolutionary similarity of tropical dry forests around the globe – more similar to each other than to nearby wet forest – is very striking” - COA plant sciences professor Dr. Susan Letcher.

“Instead of saying, ‘Here are the species that forests have in common,’ we looked at deeper levels of relatedness,” Letcher said. “We went back 100s of millions of years and instead asked, ‘Where did these trees share a common ancestor?’”

This deep look into forest genetics and the resulting understanding of shared DNA between American and African forests challenges the classic biogeographic division of Neotropics (the Americas) vs. Paleotropics (Asia and Africa). Letcher and her collaborators instead show that forests across the globe share many traits regardless of what continent they are on.

Research by Dr. Susan Letcher and others highlights common ancestry among forests across the globe.“It was not very long ago, in the evolutionary history of life, that Africa and South America separated,” Letcher said. “The evolutionary similarity of tropical dry forests around the globe– more similar to each other than to nearby wet forest– is very striking!”

Their extraordinary findings can largely be attributed to a long tradition of DNA sequencing. With the genetic code of organisms and DNA sequencing technology, taxonomists have been able to classify organisms based not just on their relatedness to each other, but on whether they have a common ancestor, and so classify organisms with their closest relatives. However, doing so on such a large global scale was not always possible.

“It is only very recently that we have had the computing power to do this with really large sets of organisms and to apply these techniques to data sets like this one. It takes huge amounts of computing power to determine which ones are more closely related to which others,” she said.

Letcher’s major contribution to the paper comprised her data set of tropical forest diversity studies gathered during her graduate work at University of Connecticut. She has always been supportive of open data sharing, and has contributed her findings to a number of repositories, she said.

The dynamics of tropical forests — especially those found in South America — are a focus of research for College of the Atlantic plant sciences professor Dr. Susan Letcher.“Collaborating with people is a great way to do science,” she said. “With my single data from one small region of Costa Rica, I can say some things about how that particular forest regenerates, but when you combine that with thousands of other points of data from hundreds of people around the world, we have a bigger lens to focus on questions of how forests are related and how they recover.”

Susan G. Letcher grew up on an island off the coast of Maine, in a house with wood heat and cold running water. After earning her B.A. at Carleton College in 2000, she hiked the Appalachian Trail twice (from Maine to Georgia and back again) with her older sister, Lucy. She completed a Ph.D. in 2008 at the University of Connecticut, studying how tropical forests recover from disturbance, and spent two and a half years teaching study abroad programs in Costa Rica.

Established in 1914, PNAS publishes cutting-edge research, science news, commentaries, perspectives, colloquium papers, reviews, and actions of the National Academy of Sciences. The journal’s content spans the biological, physical, and social sciences and is global in scope. Nearly half of all accepted papers come from authors outside the United States. PNAS strives to publish only the highest quality scientific research, and papers undergo rigorous peer review and approval by an NAS member before publication.

College of the Atlantic is the first college in the U.S. to focus on the relationship between humans and the environment. This intentionally small school of 350 students and 35 faculty members offers a Bachelors of Arts degrees in human ecology - the study of how humans interact with our naturals, social, and technological environments. Each student develops their own course of study in human ecology, collaborating and innovating across multiple disciplines. Both The Sierra Club and The Princeton Review named College of the Atlantic the #1 Green College in the United States in 2016 and 2017.

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